In general, radiation of charged particles such as alpha and beta rays, when passing through a substance, ionizes, excites, or dissociates atoms or molecules in the substance and loses energy. The energy transmitted to the substance is further emitted as thermal kinetic energy or electromagnetic waves. When the substance is, e.g., one that emits fluorescence, most of the energy is emitted as light in the visible area; and this phenomenon is referred to as scintillation and the emitted light as scintillation light.
Further, in the case of radiation carrying no electrical charge, such as gamma or neutron rays, since a similar phenomenon is also caused by secondary charged particles emitted when the radiation interacts with a substance, the radiation is detected using the scintillation phenomenon.
A photomultiplier tube is used for measuring scintillation light. The photomultiplier tube is a high sensitive photodetector added with a current amplification (=electron multiplication) function based on a phototube which converts light energy into electric energy utilizing a photoelectric effect.
Substances that causes scintillation phenomena are generally collectively called scintillators; and inorganic scintillators containing inorganic crystals typified by NaI (Tl) and organic scintillators including liquid scintillators in which fluorescent substances that emit fluorescence when radiation is incident, such as organic crystals like anthracene and terphenyl, are dissolved in organic solvents such as xylene, and plastic scintillators in which fluorescent substances are dissolved and dispersed in transparent resins such as polystyrene are used in the field of radiation measurement.
Especially, the plastic scintillators have often been used for half a century due to advantages such as their handleability and moldability into optional and large shapes.
Representative plastic scintillators are those in which first fluorescent substances such as 2,5-diphenyloxazole (DPO), 1,4-bis(5-phenyl-2-oxazole)benzene (POPOP), p-tetraphenyl (P-TP), p-quarterphenyl (P-QP) and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (B-PBD) and second fluorescent substances such as bis(O-methylstyryl)benzene (bis-MSB), 9,10-diphenylanthracene, and 9,10-dimethylanthracene are added to a styrol-based base resin such as polystyrene (PS) or polyvinyltoluene (PVT).
As longstanding common sense in the art, it has been needed to add fluorescent substances to plastic scintillators. Its principal reasons include, since the wavelengths of electromagnetic waves emitted from a base resin excited by irradiation with radiation have been reported to be as short as 150-300 nm, 1) unadaptation to the wavelength range of 300-400 nm suitable for measurement with a photomultiplier tube used for measurement and 2) the insufficient amount of light arriving at a detecting portion due to self-absorption by the base resin. In this regard, it is said that the energy of the electromagnetic waves emitted from the base resin can be converted into light of ˜350 nm with a first fluorescent substance and of ˜420 nm with a second fluorescent substance by adding the fluorescent substances to obtain scintillation light with wavelengths, suitable for measurement, having a high transmittance, and difficult to be self-absorbed.
Accordingly, it is common general technical knowledge in the art that addition of fluorescent substances to scintillators is essential, so that studies on the combinations of various base resins and fluorescent substances have been conducted for improving measurement technologies.
However, any attempt to use only a base resin as a radiation detection element has not been reported until now because of the common general technical knowledge that the addition of fluorescent substances to scintillators is essential, as mentioned above.